Tertiary Verde Formation:

The Miocene-Pliocene Verde
Formation (~ 5 million years old) is a complex
formation consisting of several different facies (distinct
units within the formation that represent different depositional
character from other units). These facies vary from limestone
to mudstone to sandstone. They are interpreted as lacustrine
(deposited in a lake) in origin, and the presence of
evaporites in the mudstone layers indicates that the
lake was periodically dry. The age of the Verde Formation
has been constrained by fossil evidence as well as K-Ar
(Potassium-Argon) dating techniques.

References:

Owen-Joyce, S.J. and Bell, C.K., 1983 Appraisal of Water Resources in the Upper Verde River Area, Yavapai County, Arizona, Arizona Department of Water Resources Bulletin 2 (Prepared by the U.S. Geological Survey), available online at: http://www.verde.org/usgs/usgsstud.html.